Biaxial orientation is a process whereby a plastic film or sheet is stretched in such a way as to orient the polymeric chains of the plastic parallel to the plane of the film but more or less randomly within this plane. Biaxially oriented films possess superior tensile properties, improved flexibility and toughness, and increased shrinkability. These improved physical properties also make possible wider utilization of the electrical and optical properties of the films.
Biaxially oriented films are produced by single direction stretching or by simultaneous stretching in both directions. Tentering consists of stretching a formed flat sheet in the transverse direction with a tenter frame within an oven. A film may also be stretched in the longitudinal direction by passing the film over rollers traveling at different speeds. Bubble blowing orientation can stretch the film simultaneously in both the longitudinal and transverse directions. Another method for stretching a film simultaneously in both the longitudinal and transverse directions is described in U.K. Patent No. GB 2,038,705A.
Stretching a film in either the longitudinal direction or the transverse direction at a temperature above its glass transition temperature (hereinafter T.sub.g) orients the film and results in a higher Young's modulus or tensile modulus in that direction. Typically, a film is subjected to a post orienting heating cycle for annealing (heat set) purposes if shrinkability is not desired.
There are a number of applications for films which have a higher modulus in one or both directions. The applications involve the use of a functional layer on the film, for example, an adhesive, an abrasive or a magnetic layer. Packaging tape having an increased longitudinal modulus provides a strong seal without the necessity of reinforcing elements in the tape. A film used as a backing for magnetic or abrasive particles, for example, a video tape or a sanding belt, requires a higher modulus in both the longitudinal direction, to prevent breakage, and the transverse direction, to prevent cupping or curling of the tape or belt.
The prior art has produced films having increased moduli in one or both directions through a series of drawing steps at temperatures typically between 10.degree. C. above the T.sub.g of the film and 40.degree. C. below melting temperature, T.sub.m, of the film. See, for example, U.S. Pat. No. 4,497,865. Multiple drawing steps often result in a lower yield as each drawing step can result in breakage of the film. A process which includes multiple drawing steps is also typically higher in cost than a process involving fewer drawing steps.
It is therefore highly desirable to provide a process for increasing the modulus of a film in a two-step process, without the necessity of multiple-drawing steps.